The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
A vehicle includes an internal combustion engine that generates drive torque. The internal combustion engine combusts an air/fuel mixture within cylinders to drive pistons that produce the drive torque. The air/fuel mixture is regulated via intake and exhaust valves. The intake valves are selectively opened to draw air into the cylinders. The air is mixed with fuel to form the air/fuel mixture. The exhaust valves are selectively opened to allow exhaust gas to exit from the cylinders after combustion of the air/fuel mixture.
A rotating camshaft of the engine regulates opening and closing of the intake and exhaust valves. The camshaft includes cam lobes that each has a profile, which is associated with a valve lift schedule. The valve lift schedule includes an amount of time a valve is open (i.e. duration) and a magnitude or degree to which the valve opens (i.e. lift).
Variable valve actuation (VVA) technology improves fuel economy, engine efficiency, and/or performance by modifying a valve lift event, timing, and duration as a function of engine operating conditions. Two-step VVA systems include variable valve assemblies such as hydraulically controlled switchable roller finger followers (SRFFs). SRFFs enable two discrete valve states (e.g. a low-lift state and a high-lift state) on the intake and/or exhaust valves. Example descriptions of the operation of SRFFs are provided in U.S. application Ser. No. 12/062,920, filed on Apr. 4, 2008, and U.S. application Ser. No. 11/943,884, filed on Nov. 21, 2007.
A control module transitions a SRFF mechanism from a low-lift state to a high-lift state and vice versa based on demanded engine speed and load. For example, an internal combustion engine operating at an elevated engine speed, such as 4,000 revolutions per minute (RPM), typically requires the SRFF mechanism to operate in a high-lift state to avoid potential hardware damage to the internal combustion engine.